1. Field of the Invention
The invention relates generally to a spin-torque oscillator (STO), and more particularly to a magnetic field sensor and sensing system that uses a STO sensor.
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer adjacent the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference layer. The other ferromagnetic layer adjacent the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the reference-layer magnetization due to the presence of an external magnetic field, such as from the recorded magnetic bits on the disk, is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as a current-perpendicular-to-the-plane (CPP) sensor.
In addition to CPP-GMR read heads, another type of CPP sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. In a CPP-GMR read head the nonmagnetic spacer layer is formed of an electrically conductive material, typically a metal such as Cu or Ag. In a CPP-TMR read head the nonmagnetic spacer layer is formed of an electrically insulating material, such as TiO2, MgO or Al2O3.
In CPP MR sensors, it is desirable to operate the sensors at a high sense current density to maximize the signal and signal-to-noise ratio (SNR). However, it is known that CPP MR sensors are susceptible to current-induced noise and instability. The spin-polarized sense current flows perpendicularly through the ferromagnetic layers and produces a spin-torque (ST) effect on the local magnetization. This can produce fluctuations of the free layer magnetization, resulting in substantial low-frequency magnetic noise if the sense current is large. More recently, it has been suggested that the ST effect can be caused by a thermal gradient wherein magnons created in an insulating ferrite are converted to an electron spin current in a ferromagnetic layer. [Slonczewski, “Initiation of spin-transfer torque by thermal transport from magnons”, Physical Review B 82. 054403 (2010)].
A spin-torque oscillator (STO) is a device that operates at a current greater than Ic to take advantage of the ST-induced forces acting on the free layer. When a fixed direct current higher than Ic is directed through the STO, a persistent oscillation of the magnetization of the free layer occurs by virtue of the ST effect. A STO sensor based on a CPP-GMR or CPP-TMR sensor has been proposed. In a STO magnetic field sensor the oscillation frequency of the free layer magnetization shifts with the application of an external magnetic field, and these frequency shifts can be used to detect changes in the external magnetic field. In a STO sensor based on a CPP-GMR or CPP-TMR sensor the current through the sensor drives the persistent oscillation of the free layer magnetization and is also used to sense frequency shifts in the free layer magnetization oscillation due to external magnetic fields. However, in a STO sensor, because the sense current should not be much greater than the threshold current, which is the current value need to cause persistent oscillation of the free layer magnetization, the output signal amplitude output is limited. US 20100328799 A1, assigned to the same assignee as this application, and US 20090201614 A1 describe proposed STO sensors for use as read heads in magnetic recording disk drives.
What is needed is a STO that enables monitoring of the oscillation frequency of the free layer magnetization and a STO sensor that that senses shifts in the frequency of oscillation of the free layer magnetization, wherein the STO and STO sensor have a high output signal amplitude not limited by the threshold current.